The usefulness of a metal deposition bath in industry depends to a large extent on the following three parameters:
absence of objectionable impurities in the deposit; PA1 the possibility of obtaining deposits with a thickness of several microns and in a reasonable period of time, which implies both autocatalytic properties and a deposition speed of several microns per hour; and PA1 long lifetime for the bath, and preferably unlimited lifetime with periodical recharging.
On the first point, French patent No. 2 590 595 in the name of the present Applicant and relating to a bath for depositing nickel and/or cobalt describes the advantage of selecting hydrazine as a reducing agent since it does not add any impurity to the deposit. That patent also indicates that numerous anions, and in particular chloride, sulfate, and nitrate anions disturb the operation of hydrazine baths.
U.S. Pat. No. 3,562,911 describes hydrazine baths for chemical deposition of platinum, said baths containing hydrochloric acid and hexachloroplatinic acid, and consequently containing chloride ions. These baths are not autocatalytic and they operate until depleted without it being possible to interrupt the reaction. The platinum efficiency is 91% at most, i.e. at least 9% of the platinum ions contained in the bath are lost.
The object of the invention is to provide an autocatalytic hydrazine bath that does not contain chloride ions and which is capable of being recharged so as to operate over long periods of time.